Permanent Magnet Moving Coil (PMMC) Instruments


Construction & Working : simply in brief , there is a coil which is free to move and is surrounded by powerfull magnets. Now we dicuss in detail ..

There is a two powerfull U-shaped magnets in the instrument. The ends of magnets have soft iron piece called pole pieces ..these iron pieces are attached to the magnets to decrease the air gap hence reluctance and to provide low reluctance path for the magnetic field produced by the permannent magnets.

Between these magnets , a coil wound on aluminium and copper core is placed which is free to rotate ..the coil is mounted on spindle to which pointer is also attached.

When current to be measured flows through the coil than two fields acts on the coil i.e. field produced by the coil current and field produced by the permannent magnets. Two field will act on the coil and causes the coil to rotate .with the coil , spindle also moves than pointer also moves through the scale and gives the necessary reading on the scale.

Advantages and disadvantages

  • Low power consumption
  • Linear scale
  • No hysteris loss
  • Powerfull eddy current damping
  • No effect of outside or external fields
  • Instrument Range can be extended with shunts and multipliers
  • High accuracy due to high torque weight ratio .

Disadvantages:

  • Costly
  • Errors due to ageing of control mechanism and magnets.

Deflecting Torque Equation

T= Force x distance

T= NBILxb Newton Meter

N= no of turns on the coil

B= flux density

L= length of coil

B= breadth of coil

I= currents flowing through coil


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